It is known that in the oxidation of olefins by means of hydroperoxides to give olefin oxides (oxiranes), the selectivity of olefin oxide formation decreases significantly and the level of undesirable secondary reactions increases as the olefin conversion rises. To be able to achieve a high selectivity of over 95% despite this, these reactions are, especially on an industrial scale, therefore preferably carried out only to an olefin conversion of about 60–95%.
Isolating the unreacted olefin from the reaction process and then recirculating it to the oxidation process is also known. Thus, a process in which a gas mixture comprising the olefin and oxygen originating from the decomposition reaction of the hydrogen peroxide used as hydroperoxide in the oxidation is separated off and the olefin is absorbed from the gas mixture in a liquid absorption medium has been proposed. In this process, a sufficient amount of an inert gas is added to the oxygen to prevent formation of flammable gas compositions (EP-B-0 719 768 B1). In a preferred embodiment, this process of EP-B-0 719 768 B1 is used for recovering propene from the reaction of propene with hydrogen peroxide to give propene oxide. The inert gas used is preferably methane and the liquid absorption medium used is a mixture comprising isopropanol and water.
Lower hydrocarbons such as heptane and octane and also methanol and acetone can also be used, but less advantageously, in this process. However, a disadvantage of the abovementioned process is that a further gas, in particular methane, in addition to the offgas stream has to be fed into the column used for the absorption. This measure is intended to prevent the formation of explosive mixtures in the gas mixture as a result of the oxygen content. The low solubility of the olefin in water-containing isopropanol is also a disadvantage in this process. For example, based on the description, it is necessary to use solvent mixtures which have a water content of from 30.6 mol % to 57.2 mol %. This high water content reduces the solubility of the olefin in the isopropanol. For this reason, relatively large amounts of solvent have to be used in order to be able to recover the olefin from the offgas stream by absorption.
EP-A-0 583 828 discloses a direct-oxidation ethylene oxide process wherein ethylene oxide resulting from the direct oxidation is absorbed and thus separated from the epoxidation product stream. As absorption agents, especially suitable are mixtures of C10 to C13 paraffins. Explicitly, dodecane and tridecan are described. Higher paraffins are described as possible but less effective. Although a process is described where a stream comprising an olefin may be compressed, EP-A-0 583 828 is silent on a process which comprises two subsequent steps, the first step being a compressing step and the second step being a cooling step. Moreover, EP-A-0 583 828 is directed to a process in which, prior to the absorption of etyhlene, carbon dioxide has to be removed from the gas stream, and the compressing step is described as being essential for removal of carbon dioxide only.
DE-AS-1 212 507 describes a method for producing alkylene oxides from alkenes such as propene using oxygen containing gases wherein the alkene is separated from a gas mixture by absorption in a hydrocarbon having a high boiling point, e.g. tetradecane. In the context of this document, it is described that a gas stream comprised of oxygen or a gas stream comprising oxygen is used for removing the alkene from the absorbing agent, the resulting gas stream being recirculated to the oxidation process. In the context of DE-AS-1 212 507, a process step is described in which a gas stream which, after a washing step, contains propene, nitrogen, carbon oxide and carbonic acid, is divided and one part is compressed and subsequently, without cooling, introduced into an absorber where the gas stream is brought in contact with diesel oil.
U.S. Pat. No. 5,599,955 discloses a method for producing propene oxide wherein, in an epoxidation zone, a propene stream is epoxidized using an aqueous hydrogen peroxide solution. An offgas stream resulting from this epoxidation containing unreacted propene is dried in a drier filled with absorbent, the resulting stream being recirculated to a olefin production zone.
WO 02/102496 A1 describes a method for the recovery of combustible components of a gas stream comprising the combustible components and oxygen by selective absorption of the combustible components in a solvent. As combustible components, propene oxide and propene are mentioned. As suitable solvents, usual organic solvents such as alcohols, aromatic and aliphatic hydrocarbons or ketones are generically disclosed. As preferred solvent, compounds are described the gas stream is derived from prior to the absorption. According to WO 02/102496 A1, the gas stream comprising combustible components is directly introduced into the absorption unit, and neither a step of compressing nor a step of cooling is mentioned.
It is an object of the present invention to provide an improved method for separating an olefin from a gas stream, said gas stream comprising the olefin and at least one other component, by means of which it is possible to achieve a more effective recovery of the olefin said the gas stream than in the process of the prior art.
It is a further object of the present invention to provide an improved process for separating an olefin from a gas stream, said gas stream additionally comprising oxygen, by an absorption process wherein no additional inert gas has to be added to the absorption unit used.